1. Field of the Invention
In the present state of the art, in order to measure linear quantities with micrometric accurancy and discrimination, it is known to use, inter alia, linear measurement transducers mainly based on the following incremental measuring systems:
(a) interferometric, by means of a laser beam, in which the size of the incremental measurement unit is determined in relation and proportion to the wavelength of the beam used; PA0 (b) inductive and/or capacitive, in which the size of the incremental measurement unit is generally obtained by measurement and sinusoidal interpolation of the intensity of the high frequency electrical or magnetic field produced by a linear inductor having its poles spaced apart by a length equal to a multiple of the size of the incremental measurement unit; PA0 (c) optoelectronic, using an optical rule, in which the size of the incremental measurement unit is determined by equidistant micrometric notches engraved on a glass or metal scale at a spacing equal to the measurement unit or a multiple thereof, and such as to be able to be intercepted optoelectronically. PA0 (a) interferometric system: usable for measurements up to and beyond 30 meters, generally with a discrimination of 1 .mu.m or less; PA0 (b) induction system: usable for measurements up to 10 meters with a discrimination of 10 .mu.m and 5 .mu.m, and for shorter lengths with a discrimination of up to 1 .mu.m; PA0 (c) optoelectronic system: usable for measurements up to 20 meters with a discrimination of 10 .mu.m and/or 5 .mu.m, and for lengths up to 3 meters with a discrimination of 1 .mu.m.
For industrial use, such linear measurement transducer are classified as follows in terms of their particular application characteristics:
2. Description of the Prior Art.
The optoelectronic system--which uses glass or metal optical scales--is at present applied industrially to a greater extent than the other two systems, in particular in the field of machine tools and manufacturing machines in general.
In this measurement system, the measurement rule is universally constructed, as already stated, with a notched scale consituted by a precision-machined mechanically rigid glass or metal bar, with micrometric notches disposed transversely to its longitudinal axis, and such as to allow--by sensing, by transparency or reflection, the moire fringes produced by optical interference with an equivalent grid of notches on the reading cursor--the formation of electrical pulses of a number equal to the number of incremental units scanned over the length travelled by the reading cursor, and of concordant phase with the direction of measurement.
More particularly, in measuring systems of this type, to which the present invention relates, the transducer comprises two functionally inseparable essential parts, namely:
the measurement scale, consituted by a perfectly rectilinear element, of which the surface carries the engraved measurement notches which can be sensed optically either by transparency or by reflection, and
the reading head, which is mobile along the scale and is arranged to intercept the passage of the scale notches by transparency or optical reflection, and to provide electrical pulses of a sequence and phase which are concordant with the relative movement between scale and the head, these pulses then being used for controlling bidirectional electronic counters and/or apparatus for monitoring and controlling machine tools, such as "dimensions display units", "positioners", "numerical controls" and the like.
According to the known art, nearly all measurement transducers using linear scales of the engraved notch type are constructed with a glass or stainless steel scale in the form of a rectilinear bar of rigid structure. This bar has a fairly large cross-section to enable it to be fixed by screw means or the like over its entire length in a seat provided in a so-called support rule, this generally being in the form of an extruded aluminium section, and serving as a support and slide guide for the reading head.
The steel or glass bar which forms the scale has generally a rectangular cross-section &gt;&gt;10 mm.sup.2, so as to be mechanically rigid, as already stated. In this respect, it must also act as the guide for the reading head in cooperation with the support rule over the entire length of said scale, so as to ensure precise alignment of the head with the scale axis, both in the azimuthal and zenithal directions.
In the present state of the art it is not economically possible to manufacture linear transducers with a glass or steel scale for lengths exceeding 2 or 3 meters without any discontinuity, and such transducers are therefore constructed by fixing two or more aligned bars on to the support rule.
This scale alignment method involves technological and production difficulties, and manufacturing costs which are greater the greater the length of the transducer.
In order to obviate thse difficulties, it has already been proposed to engrave the measurement notches on a flexible steel strip which is not under tension, and to then glue this strip on to a rigid support bar.
This obviously simplifies manufacture from the point of view of maintaining an exact distance between the notches over the entire length of the scale. However, the support bar must be again manufactured in the form of several precision-machined pieces, of which the assembly involves the same alignment difficulties, with consequent formation of undulations in the strip which can then generate reading errors.